1. Field of the Invention
The present invention relates to an antenna and a tag having the antenna mounted thereon, and more particularly, an antenna constituted of a loop antenna suitable for a non-contact tag performing transmission and reception to/from an RFID reader, and an RFID tag having the antenna mounted thereon.
2. Description of the Related Art
In recent years, introduction of a data management system has been spreading, in which a tag storing commodity information is attached to a commodity, including a living creature such as farm animal, and the information is read in a non-contact form.
A system called RF (Radio Frequency) ID system is one example of such a system. In the RFID system, a signal of approximately 1 watt is transmitted from a reader/writer, using a UHF band (860-960 MHz) radio signal.
On the tag side, based on the received signal, power for initiation is generated, and in response to the instruction information included in the signal, a response signal is returned from the tag side to the reader/writer side. With this, the reader/writer side can read information stored in the tag.
A tag attached to the commodity is constituted of an antenna and a functional chip connected to the antenna. A communication distance between the tag and the reader/writer is nearly 3 meters, though it depends on an antenna gain embedded in the tag, an operating voltage of the chip and ambient environment.
Here, an interface section in an LSI chip 20 mounted on the tag can equivalently be represented by a parallel connection of a resistor Rc (for example, the resistance=1000 Ω) and a capacitor C (for example, the capacitance=0.7 pF), as shown in FIG. 1 in which matching between the antenna and the LSI chip is illustrated. Meanwhile, an antenna 1 mounted on the tag (which is called tag antenna) can equivalently be represented by a parallel connection of a resistor Ra (for example, the resistance=1000 Ω) and an inductor L (for example, the inductance=40 nH).
By connecting the above both connections in parallel, the capacitance C and the inductance L resonate. As can be seen from formula (1) shown below, an imaginary component becomes zero. This results in a match, and thereby the reception power in a tag antenna 10 is sufficiently supplied to the LSI chip 20 side.f=½π√LC  (1)
Usually, a most basic form of tag antenna 10 is a resonant dipole antenna having a total length of λ/2. In the UHF band, a necessary length is approximately 150 mm. However, when considering a case of, for example, managing a farm animal by pasting a tag onto an ear thereof, the tag must be formed with a diameter of the order of 35 mm.
However, it is hardly possible to form a λ/2 resonant dipole antenna of such a required size.
Accordingly, as shown in FIG. 2, it is considered to use a loop antenna constituted of a loop antenna 1 being connected to a feeder terminal 3. The loop antenna has a characteristic which is described in, for example, Antenna Kogaku Handbook, pp 20-22 (Kogaku means Engineering; published by Ohmsha, Ltd. on 5 Mar. 1999). Namely, it is explained that, under a condition that a unique current of an identical phase flows in the circle-shaped loop 1 being connected to feeder terminal 3, the larger the radius of the loop becomes, the more the number of robes in the power pattern increases.
Now, as described above, in a tag for use in an RFID system, the most important point in regard to matching between chip 20 and tag antenna 10 is that the capacitance C in the interface section on LSI chip 20 side and the inductance L of tag antenna 10 resonate in a use frequency, as shown in FIG. 1.
In contrast, when tag antenna 10 is structured of only a loop antenna as shown in FIG. 2, since there is no part of the inductor L such as shown in FIG. 1, matching with the interface section of LSI chip 20 cannot be made satisfactorily. As a result, power is not supplied sufficiently from tag antenna 10 to the chip 20 side. This produces an inconvenience of an extremely decreased communication distance.
Further, tag antenna 10 may be used as a unit by being housed into a dielectric case for the sake of handling convenience. Therefore, when determining the antenna shape, it is necessary to deeply consider permittivity of the dielectric for use as the case, and the thickness thereof.